Several groups and individuals are credited with the independent discovery of carbon nanotubes. The most influential papers identifying the basic structure of carbon nanotubes are arguably the paper by Iijima (1991) for multiwall carbon nanotubes, and the 1993 papers by Iijima and Ichihashi (1993) and by Bethune et al. (1993), which describe single-wall carbon nanotubes. A detailed discussion of the discoverers of carbon nanotubes can be found in Monthioux and Kuznetsov (2006).
Since Iijima's 1991 paper, carbon nanotubes have been the subject of intense theoretical and technological interest in materials science because of their extreme characteristics. Their theoretically-predicted and experimentally-measured mechanical properties, including high strength, high stiffness, toughness, and low density, should make them ideal strengthening material in advanced fiber-reinforced composites, woven fabrics, and textiles with potential applications in aeronautics, automotive systems, armor, and civil engineering. Both theoretical and experimental studies have shown that the elastic modulus of a carbon nanotube is in the range of 1-2 TPa (Krishnan et al., 1998). Treacy et al. (1996) measured the Young's modulus of isolated nanotubes by measuring the amplitude of their intrinsic thermal vibrations using transmission electron microscopy, and found the average value of Young's modulus to be 1.8 TPa. Haskins et al. (2007) used Tight-Binding Molecular Dynamics simulations to examine the effects of defects on carbon nanotubes, and deduced values of Young's modulus and tensile strengths for (5,5) chiral carbon nanotubes of between 0.95 to 1.15 TPa and 70 to 110 GPa, respectively, for the molecular detects considered. Dresselhaus et al. (2008) provide a fairly recent and comprehensive summary of the research results of the properties of carbon nanotubes.
Carbon nanotubes are produced in primarily two configurations; single-walled carbon nanotubes (SWNT) and multi-walled carbon nanotubes (MWNT), and can be twisted or woven into carbon nanotube fibers. Parallel-aligned carbon nanotube fibers form hexagonal closest packed fibers because of the van der Waals forces of attraction between the individual carbon nanotubes. While the strength and stiffness of carbon nanotubes are extremely high, to date fibers of aligned carbon nanotubes have been found to be far weaker than the constituent carbon nanotubes (e.g., Qian et al., 2003; Ericson et al., 2004; Koziol et al., 2007; and Zhang et al., 2008). There is evidence that slippage between overlapping carbon nanotubes occurs in parallel aligned carbon nanotube fibers and MWNTs under strain and that the slippage occurs at tensions well below the breaking strength of the carbon nanotubes (Walters et al., 1909; Yu et al., 2000a, 2000b; Qian et al., 2003). There are several estimates in the literature for the carbon nanotube contact length required to produce a frictional force equal to the breaking strength of the carbon nanotubes. Qian et al. (2002) estimate that the carbon nanotube contact length required to achieve the load transfer needed to reach the intrinsic carbon nanotube breaking strength could be on the order of 10-120 μm. Yakobson et al. (2000) estimated that the CNT contact length needed for fibers to approach a full strength of carbon nanotubes was on the order of 10 μm. Twisting and stretching the individual strands into a fiber has been demonstrated to increase the load transfer between the carbon nanotubes and result in higher elastic modulus and strength Walters et al., 1999; Qian et al., 2003; Liu and Qin, 2005, Zhang et al., 2004; Zhang et al., 2007; Koziol et al., 2007; Kleis et al., 2008; Zhang and Li, 2009). However, the tensile strength of the twisted fibers is still considerably less than the tensile strength of the constituent carbon nanotubes.
Simulations of neat fibers of aligned carbon nanotubes were performed to determine the stress-strain characteristics of parallel-aligned carbon nanotube fibers. The simulations were carried out using Sandia Laboratory's Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) code (Plimpton, 1995). The potential energy of the system was calculated using the Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) Potential (Stuart et al., 2000). Tersoff type potentials such as AIREBO are reactive meaning they allow bonds to form and break during the course of a simulation. They have been shown to be qualitatively good at modeling the mechanical properties of carbon-based materials (Yakobson et al., 1996; Yakobson et al., 1997; Cornwell and Wille, 1997; Garg et al., 1998). The computational cost of reactive potentials is relatively high compared to non-reactive potentials. Simulating large ensembles of carbon nanotubes containing millions of atoms requires high performance computer (HPC) and computer algorithms that scale efficiently on thousands of processors LAMMPS is specifically designed to run on parallel computer systems and has good scaling characteristics on a wide range of HPC platforms (Sandia National laboratories, no date). The calculations were performed on the Cray XT3 (Jade) at the DoD Supercomputing Resource Center (DSRC) located at the U.S. Army Engineer Research and Development Center, Vicksburg, Miss.
Initial simulations (Cornwell et al., 2009) indicate that the factor limiting the strength of the parallel-aligned carbon nanotube fibers is the poor load transfer between the carbon nanotubes. To overcome this limitation, covalent interstitial carbon atoms (that is, a carbon atom inserted between the strands of the fiber) were introduced. The interstitial carbon atoms form chemical bonds between the strands of the fibers to facilitate load transfer between the carbon nanotubes and thus prevent slipping. Such chemical bonds have been observed experimentally by several groups (e.g., Kis et al., 2004; Krasheninnikov et al., 2003; Sammalkorpi et al., 2005; Pregler and Sinnott, 2006; Peng et al., 2008) and were created as a result of irradiation of carbon nanotubes with high-energy particles.